c++ - Returning to the menu & exiting the program -

(c++) i'm writing program assignment , requires program prompt user return menu or quit program after each case, problem i'm not sure how implement efficiently. ideas? quitting, call last function called 'exit'

 #include <iostream>                                                                //libraries   using std::cout; using std::cin; using std::endl;  int menu(double pi); int circlearea(double pi); int circlecircum(double pi);                                                    //function declarations demanded shapes, passing pi in order reuse in both circle calculations int rectanarea(); int triangarea(); int cubvol(); void exit();  int main() {     double pi = 3.14159265359;                                                  //declaration of pi using in circles (passed)     menu(pi);     system("pause");     return 0; }  int menu(double pi)                                                             //menu choosing shape {     int choice = 0;     cout << "shape calculator created by:\n\nplease select wish calculate:\n\n1 - area of circle\n\n2 - circumference of circle\n\n3 - area of rectangle\n\n4 - area of triangle\n\n5 - volume of cuboid\n\n ";     cin >> choice;     system("cls");       switch (choice)                                                                //switch case each shape         {         case 1:             circlearea(pi);             break;         case 2:             circlecircum(pi);             break;         case 3:             rectanarea();             break;         case 4:             triangarea();             break;         case 5:             cubvol();             break;         default:             cout << "invalid input! please try again.\n\n";             break;         }     return 0; }  int circlearea(double pi)                                                       //the area of circle function {     double radius = 0;      cout << "please enter radius of circle:\n\n";                      //asks radius first     cin >> radius;     system("cls");      cout << "the area of circle is:\n\n" << radius*radius*pi << "cm/2\n\n";                                //display calculation calculated using user input, formula pi*radius(squared) reversed     return 0; }   int circlecircum(double pi)                                                     //the circumference of circle function {     double radius = 0.0;      cout << "please enter radius of circle:\n\n";                      //asks radius first     cin >> radius;     system("cls");      cout << "the circumference of circle is:\n\n" << pi*radius * 2 << "cm\n\n";                            //calculates circumference using user input, formula 2*pi*radius reversed     return 0; }  int rectanarea()                                                                //function area of rectangle {     double rectanheight = 0.0;     double rectanwidth = 0.0;      cout << "please enter height of rectangle:\n\n";                   //asks input height first     cin >> rectanheight;     system("cls");      cout << "height = " << rectanheight << endl << endl << "please enter width of rectangle\n\n";      //shows inputted height above, asks input of width     cin >> rectanwidth;     system("cls");      cout << "the area of rectangle is:\n\n" << rectanheight*rectanwidth << "cm/2\n\n";                     //calculates area of rectangle using input, formula height*width displays     return 0; }  int triangarea()                                                                //function area of triangle {     double triangbase = 0.0;     double triangheight = 0.0;      cout << "please enter base measurement of triangle\n\n";           //asks input of base first     cin >> triangbase;       system("cls");      cout << "base = " << triangbase << endl << endl << "please enter height of triangle\n\n";          //displays inputted base , asks height     cin >> triangheight;     system("cls");      cout << "the area of triangle is:\n\n" << triangbase*triangheight / 2 << "cm/2\n\n";                   //calculates area using user input , displays it, formula base*height/2     return 0; }  int cubvol()                                                                    //function volume of cuboid {     double cublength = 0.0;     double cubwidth = 0.0;     double cubheight = 0.0;      cout << "please enter length of cuboid\n\n";                       //asks input of length first     cin >> cublength;     system("cls");      cout << "length = " << cublength << endl << endl << "please enter width of cuboid\n\n";            //displays inputted length , asks width     cin >> cubwidth;     system("cls");      cout << "length = " << cublength << endl << "width = " << cubwidth << endl << endl << "please enter height of cuboid:\n\n";        //displays inputted length , width, asks height     cin >> cubheight;     system("cls");      cout << "the volume of cuboid is:\n\n" << cublength*cubwidth*cubheight << "cm/3\n\n";                  //displays calculation using inputted information, formula length*width*height     return 0;  }  void exit() {         cout << "**************************************" << endl;         cout << "**************************************" << endl;         cout << "*******    t h n k  y o u    *******" << endl;         cout << "*******    f o r  u s n g    *******" << endl;         cout << "******* t h s  p r o g r m *******" << endl;         cout << "**************************************" << endl;         cout << "**************************************" << endl; } 

in main, substitute menu(pi) with:

char choice; bool exitnow = false;  do{     menu(pi);     cout<<"return (r) or quit (q)?"<<endl;     cin>>choice;     if(choice == 'q')         exitnow = true; } while (!exitnow); exit(); 

it should work if haven't tested it.

edit: code above doesn't check if correct input entered. should solve:

char choice = 0; bool exitnow = false;  do{     menu(pi);      do{         cout<<"return (r) or quit (q)?"<<endl;         cin>>choice;     }     while( !(choice == 'r' || choice == 'q') && cout<<"invalid input!"<<endl);      if(choice == 'q')         exitnow = true; } while (!exitnow); exit();